The notes you quote are from 2016.
The Engine 4 issues were significant enough to cause many players to quit for a while. I was one of those, and I'd love to hear some discussion of how the problems were addressed.

I know fire isn't the CM engine's strongest feature, but it does exist. Sometimes you'll see that a building's interior shows flames on the floor after it has taken damage.
What is the actual effect of these fiery floors? Will units in them take damage? Can my units enter them safely or not? I would assume that they are off limits, but it's certainly possible to order units to dance on the coals.
Any enlightenment?

I know nothing of how they're actually used, but would that be done in combat? (The behavior of the crew up top suggests that they're not worried about coming under fire.)
Slowly crawling through a wall like like (or, more likely, pushing it down into a rubble pile and then backing away for the dismounts to clamber over) seems like a good way to get RPG'd by whoever is on the other side.
In any case, yeah, CM doesn't yet model all potential vehicle behavior, most especially vehicles that might be used to clear mines and breach walls.

Engine 4.0 is the latest.
Unfortunately, in the WW2 titles it introduced some issues around artillery and retreat behavior that seem unrealistic and unplayable. (For example, even crack and elite units in cover will stand up and retreat under artillery fire.)
When people talk about the upcoming "4.0 patch," they're talking about a patch that will correct these issues in games already updated to the 4.0 engine. So far, only CMSF2 is at that point.

1) Since detecting mines often requires several turns (i.e. it's not automatic), a series of waypoints will probably miss them.
2) "Mark Mines" only becomes active when a unit capable of marking mines is on or adjacent to a detected mine square.
3) Right under the vehicle description it says "Note: The specialized engineering capabilities of this vehicle are not currently simulated." Currently, only foot engineer types can mark mines.
4) There is no reliable way to eliminate a minefield in the game. Marking them makes it somewhat safer to pass through (do it slowly), but they can still go off. Even the trick of artillery bombardment on a minefield might leave some active.

They try, but under battlefield conditions (and over the course of a few minutes rather than an hour) it's probably easy to miss some threats.
I feel sure that I once did a test where I ran vehicles over marked and unmarked AT mines. Vehicles with a "slow" move order did notably better through marked AT mines, although it's still possible to lose them.
I also seem to recall that mines can have an effect on adjacent tiles, so clearing a one-tile lane isn't good enough if there is a line of mined tiles.

Just something I noticed:
I have been happily playing the first mission of the Task Force Thunder campaign, restarting a couple of times to try different openings, and yesterday I noticed something weird.
I had started a new campaign, then (I forget why) quit out to main menu and started again (turn-based, Iron mode like always). This time, however, I noticed right from the beginning that all of the trenches were in the wrong place. If you're an old CM hand, you know exactly how the trenches in that first mission are laid out, but this time I could see that they were all at a weird diagonal from the usual setup, a series of parallel diagonal lines (like "/////") across the map rather than the usual pattern.
At first I wondered whether this was some new variable setup added by engine upgrades, but it quickly became clear that it was just a messy glitch. Units also seemed to be out of place.
I exited out of the game (to desktop, not to main menu) and started the campaign again. This time all was as expected, and I have been playing with no further issues.
Anyone else ever notice anything like this? I report it just because it might be a new issue.

And the Mark Mines move order is only available when the engineers have actually detected mines. That is, the move menu is not a magic mine detector in itself.
Sometimes engineers have to sit in place for three or more minutes before adjacent mines are even detected (and thus available to be marked).

This is where those .50 cals and grenade launchers on your APCs are so helpful.
Pouring 30 seconds of such fire into a building can heavily suppress everyone looking your way, even better if you can target different floors.

Often a simple Hunt will give the enemy the opportunity to gun down your troops as they enter the doorway. The rest will hug the ground outside and be easy prey.
I haven't downloaded the new demo, but in my view the best CMx2 way to enter a building known or suspected of containing enemies is to Fast/Quick move to the outside wall. At that waypoint, give a 10 or 20 second pause along with a Area Target order aimed inside the building. Then set a Quick waypoint into the building itself, applying a new Facing order (which will serve as a cease fire, although units will of course fire on their own if they spot more enemies).
This will have the effect of rushing the building exterior and then pouring fire (and, crucially, hand grenades) inside through windows and doors before entering. It's even better if you can time it so that another squad or team provides suppressing fire up to about 5 or 10 seconds before your entry team arrives at the outside wall.
Obviously this method is expensive in terms of ammo and grenades, but it's the safest way to get inside. You have decide whether the expense is worth it.

When I look in the list of my Battlefront orders, I see everything listed from the CMSF Marines module forward, but my original copy of CMSF (which I did order from Battlefront) is not in my list.
I still have the original physical copy and the license key. Should I be worried that my key won't work if it's not in my orders list?

Such great news! $35 armed and loaded.
Quick idle question:
I own all the original CMSF content as DVD-ROMs from Battlefront. Will the upgrade be like a patch to those installations, or will it (as I'm guessing) be a Big Bundle download in the style of the recent Engine 4.0 CM releases?

I've made the point several times, but for me the 4.0 issue is experienced troops leaving cover and running when (in previous engines) they would instead briefly cower, simulating just keeping their heads down.
I've had fresh elite and crack British paras run out of heavy buildings when they came under rifle or very distant MG fire for less than a minute. Something has definitely changed.